Electrodes from fluid coke



Unite ELECTRODES FROM FLUlD COKE Fred W. Barres, Westfield, and James H.McAteer, Cranford, N. 3., assignors to Esso Research and EngmeermgCompany, a corporation of Delaware No Drawing. Application Qctober 22,1954,

Serial No. 464,158

2 Claims. (Cl. 204-294) nature which can be utilized for the obtainingof aluminum from its ores.

In the manufacture of aluminum by electrolytic reduction of alumina in asuitable fused bath, the necessary carbon electrodes have usually beenmanufactured from so-called petroleum coke, of relatively high purity.

This petroleum coke had been obtained largely from coking processes suchas delayed coking which provide particles of relatively large diameterand densities. It was thus thought that a particle size distribution ofless than 200 mesh to up to and high true or particle densities, i. e. 2or higher, were required for satisfactory electrodes. The principalcriteria of these finished electrodes are a minimum compression strengthof 4400 pounds per square inch, a minimum real density of about 1.45 anda maximum resistivity of 3 l0- ohrn-inch. As stated it was believed thatan aggregate containing large particles of calcined coke were necessaryto obtain these characteristics.

There has recently been developed an improved process known as the fluidcoking process for the production of fluid coke and the thermalconversion of heavy hydrocarbon oils to lighter fractions. The fluidcoking unit consists basically of a reaction vessel or coker and aheater or burner vessel. In a typical operation the heavy oil to beprocessed is injected into the reaction vessel containing a denseturbulent fluidized bed of hot inert solid particles, preferably cokeparticles. Uniform temperature exists in the coking bed. Uniform mixingin the bed results in virtually isothermal conditions and effectsinstantaneous distribution of the feed stock. In the reaction zone thefeed stock is partially vaporized-and partially cracked. Product vaporsare removed from the coking vessel and sent to a fractionator for therecovery of gas and light distillates therefrom. Any heavy bottoms isusually returned to the coking vessel. The coke produced in the processremains in the bed coated on the solid particles. Stripping steam isinjected into the stripper to remove oil from the coke particles priorto the passage of the coke to the burner.

The heat for carrying out the endothermic coking reaction is generatedin the burner vessel. A stream of coke is transferred from the reactorto the burner vessel employing a standpipe and riser system; air beingsupplied, to the riser for conveying the solids to the burner.Suflicient coke or carbonaceous matter is burned in the burning vesselto bring the solids therein up to a temperature suflicient to maintainthe system in heat balance. The burner solids are maintained at a highertemperature than the solids in the reactor. About 5 %l5 of coke, basedon the feed, is burned for this purpose. The unburned portion of thecoke represents the net coke formed in the process and is withdrawn.

ea-ivy hydrocarbon oil feeds suitable for the coking process are heavyor reduced crudes, vacuum bottoms,

ates Patent ice pitch, asphalt, other heavy hydrocarbon petroleumresidua or mixtures thereof. Typically, such feeds can have an initialboiling point of about 700 F. or higher, an A; P. I. gravity of about 0to and a Conradson carbon residue content of about 5 to wt. percent. (Asto Conradson carbon residue see ASTM Test D-1O-52.) It is preferred tooperate with solids having a particle size ranging between 100 and 1000microns in diameter with a preferred average particle size range between150 and 400 microns. Preferably not more than 5% has a particle sizebelow about 75' microns, since small particles tend to agglomerate orare swept out of the system with the gases.

The method of fluid solids circulation described above is well known inthe prior art. Solids handling technique is described broadly in PackiePatent 2,589,124, issued March 11, 1952.

The fluid coke product is laminar in structure and may comprise some 30to 100 superposed layers of coke. The size distribution is such that apredominant portion, i. e., about 90 weight percent has a diametersmaller than 400 microns with a range of about75 to 850 microns. Thereal density of these coke particles after the required calcining is inthe range of 1.83 to 1.93, preferably 1.87 to 1.92.

It had consequently been thought that this fluid coke because of thesmall size and low densities was not adapted for fabrication into thedesired carbon electrodes. The laminar and largely spherical structurewas also considered a possible disadvantage. It is impractical on theother hand to modify the fluid coking process to obtain larger particlesas these would be inconsistent with proper fluidization of the cokerbed.

It has now been found that this fluid coke can be utilized inthe'preparation of the indicated carbon electrodes. This is accomplishedby calcining the fluid coke to a real density in the range of 1.83 to1.93, preferably 1.87 to 1.92 and a resistivity of 2030 10* ohm-inch.This resistivity is determined at 500 p. s. i. on a sample of coke, 1sq.-inch cross-sectional area by 1 inch length, comprising material of210 to 420 microns diameter. The coke is ground so that 20 to weightpercent of the total fluid coke charge to the electrode manufacturingstep has a diameter of less than microns. The calcining preferablyprecedes the grinding but can follow the latter. The fluid coke chargeis thereafter processed into electrodes in the conventional manner. Thefinished electrodes have a minimum real density of about 1.45 and aresistivity of. below.3 10-' ohm-inch.

It is surprising to find that the fluid coke can be so utilizedespecially in view of the fact that it requires additional grinding ofparticles all of which are in a size range which ordinarily wouldrepresent only a very small fraction of the coke aggregate. As a matterof fact data establish that in the absence of this grinding,unsatisfactory electrodes are prepared.

The calcining of the fluid coke is performed in the conventional manner,i. e., a calcination at a temperature in the range of 2000- to 2800 F.or higher. This can be done in a fluid moving or fixed bed in thepresence of an atmosphere such as air,- nitrogen, carbon dioxide,hydrogen, etc. The calcination is conducted until a real density in therange of 1.83-1.93 preferably 1.87- 1.92 is obtained. Thetime necessaryis thus in the range of 0.5 to '10 hours. Longer calcining times may beused, especially in the lower temperature range, without deleteriouseffects.

The grinding of the coke particles can be accomplished in theconventional manner. It is to be understood that 0 the term grinding asused herein connotes generically any means of mechanically diminishingthe particle sizes and includes high velocity attrition, ball and millgrinding etc.

In the manufacture of the electrode itself the coke is admixed with andcharged together with a carbonaceous binder to the fabrication system.The binders utilized are conventional and include materials such as thearomatic coal tar pitch binders e. g. see U. S. Patent No. 2,683,107.Such binders generally have melting points lying within the range of70-120 C. They contain small amounts of hydrogen (about or less). Theconcentration of benzene and nitrobenzne insoluble portions representpreferably about 20-35% and 5-15%, respectively, of the binder. Thebinder is utilized in an amount of about 18 to 45 parts by weight per100 parts of fluid coke.

In general, two types of electrodes are employed by the industry (a)prebaked and (b) Soderberg self-baking electrode. In the former, amixture comprising about 78- 82% of calcined coke aggregate and 18-22%of coal tar pitch is molded at pressures of about 3000-5000 p. s. i.

or extruded and then baked for periods up to 30 days at 1800-2400 F.These preformed electrodes are then used in electrolytic cells, beingslowly lowered into the molten alumina as they are consumed, Butts ofthe unconsumed electrodes are reground and used in subsequent electrodepreparations.

The Soderberg process involves the continuous or intermittent additionof a coke-coal tar pitch paste to the top of the cell as the electrodecomponents in the lower part of the cell are consumed. In this operationthe paste represents a blend of about 70-72% coke aggregate and 28-30%of pitch. The cells operate usually at temperatures of 1700l900 F. andelectrodes are consumed at the rate of about 0.5 to 1.0 inch per day.The paste is baked into an electrode by the hot cell gases in the periodbetween the time it is added at the top and time it is used. The netconsumptionof coke represents 0.4 to 0.7 lb. per pound of aluminum metalproduced. It can be seen that the actual manner of fabricating the el'ectrodes is not the essence of this invention. Both methods have in commonthe baking of the mixed fluid coke and binder at a temperature intherange of l700 to 2400 F. 7

This invention and its advantages will be better illustrated by thefollowing examples of electrodes prepared in the manner taught.

EXAMPLE 1 A sample of fluid coke was calcined in air at about 2000 F.until a real density of about 1.9 was obtained. The particle sizedistribution after'grinding was approximately 50% in the 75-850 micron.rangeand 50 wt. percent smaller than .75 microns. Seventy parts of thisaggregate was mixed with about 30 parts lay-weight of coal tar pitch and.the paste baked in the mold for 4 days. The temperature was increasedregularly up to 1800" F. over the first three days and was held at 1832F. for the remaining time. The baked electrode had a compressionstrength of 4600 pounds per square inch and an electrical resistivity of2.6 to 2.8Xl0- ohm-inch. In these and other characteristics the productsmore than meetthe specifications for a satisfactory product.

EXALIPLEZ' crons, and the third con-taining60 wt; percent of particlessmaller than 75 microns. Thus, only the second sample was within theprocess taught by this invention. All three samples. were then molded at300 p'. s. i'. ,g. in-a charge of f8 wt. percent coal tar pitch and 82wt. percent coke aggregates, all other conditions being kept the same.The results are presented below:

TABLE Baked carbon electrode properties Wt. percent of 75 microns andsmaller particles in coke aggregate 0 30 60 Density, g./om.= 1. 36 1.48 1. 40 Resistivity, ohm-imXlO-L- 3. 59 2. 57 3. 02 CompressionStrength, p. s. l 6, 200 6, 200 7, 600

It should be noted from this example how only the sample prepared with30 wt. percent of the smaller particles, or within the 20 to 50% rangetaught by this invention, satisfactorily meets the requirements in termsof density and resistivity of the finished product.

In order to give more details on the preparation of fluid coke, thefollowing conditions of operation of the fluid coker are set forthbelow.

i These electrodes find greatest utility in their use as anodes for theobtaining of aluminum from its ores by the electrolytic process. Theprinciples involved can be utilized however in the preparation of otherelectrodes. It is to be understood that this invention is not limited tothe specific examples which have been oiiered merely as illustrationsand that modification may be made without departing from the spirit ofthe invention.

What is claimed is: r

1. A method of making a carbon electrode having a minimum real densityof about 1.45, a maximum resistivity of 3 10- ohm-inch and a minimumcompression strength of 4400 p. s. i. from a charge of fluid cokeparticles having a diameter distribution in the range of about -850microns with about weight percent having a diameter smaller than 400microns said fluid coke particles having been produced by contacting aheavy petroleum oil coking charge stock at a coking temperature with abody of fluidized coke particles in a reaction zone wherein the oil isconverted to product vapors and carbonaceous solids are continuouslydeposited on the coke particles, removing product vapors from the cokingzone, heating a portion of the coke particles from the coking zone in aheating zone to increase the temperature of said fluidized particles,returning a portion of the heated coke particles from the heatingzone'to the coking zone and withdrdawing coke product particles, whichcomprises the steps of calcining the fluid coke particles to arealdensity of 1.87-1.92; grinding the calcined fluid coke particles so that20-50 weight percent ofthe fluid coke charge has a diameter of less than75 microns; admixing the ground fluid coke with a carbonaceous binder inthe ratio of approximately 18-45 parts by wt. per parts of fluid coke;and baking the mixture at a temperature in the range of 1700-2400 F.

2. A method of making a carbon electrode having a minimum real densityof about 1.45, a maximum resist'ivity of 3x10 ohm-inch and a minimumcompression strength of 4400 p. s. i. from a charge of fluid cokeparticles having a diameter distribution in the range of about 75-850microns with about 90 weight percent having a diameter smaller than 400microns'said fluid coke particles having been produced by contacting aheavy petroleum oil coking charge stock at a coking temperature with abody of fluidized coke particles in a reaction;

zone wherein the oil is converted to product vapors and carbonaceoussolids are continuously deposited on the coke particles, removingproduct vapors from the coking zone, heating a portion of the cokeparticles from the coking zone in a heating zone to increase thetemperature of said fluidized particles, returning a portion of theheated coke particles from the heating zone to the coking zone andwithdrawing coke product particles, which comprises the steps ofgrinding the fluid coke particles so that 20-50 weight percent of thefluid coke charge has a diameter of less than 75 microns; calcining thefluid coke particles to a real density of 1.87-

1.92; admixing the ground fluid coke with a carbonaceous binder in theratio of approximately 18-45 parts by weight per 100 parts of fluidcoke; and baking the mixture at a temperature in the range of 1700-2400F.

References Cited in the file of this patent UNITED STATES PATENTS2,600,078 Schutte June 10, 1952 10 2,700,642 -Mattox Jan. 25, 1955FOREIGN PATENTS 491,522 Canada Mar. 24, 1953

1. A METHOD OF MAKING A CARBON ELECTRODE HAVING A MINIMUM REAL DENSITY OF ABOUT 1.45, A MAXIMUM RESISTIVITY OF 3X10-3 OHMI-INCH AND A MINIMUM COMPRESSION STRENGTH OF 4400 P. S. I. FROM A CHARGE OF FLUID COKE PARTICLES HAVING A DIAMETER DISTRIBUTING IN THE RANGE OF ABOUT 75-850 MICRONS WITH ABOUT 90 WEIGHT PRECENT HAVING A DIAMETER SMALLER THAN 400 MICRONS SAID FLUID COKE PARTICLES HAVING BEEN PRODUCED BY CONTACTING A HEAVY PETROLEUM AOIL COKING CHARGE STOCK AT A COKING TEMPERATURE WITH A BODY OF FLUIDED COKE PARTICLES IN A REACTION ZONE WHEREIN THE OIL IS CONVERTED TO PRODUCT VAPORS AND CARBONACEOUS SOLIDS ARE CONTINUOUSLY DEPOSITED ON THE COKE PARTICLES, REMOVING PRODUCT VAPORS FROM THE COKING ZONE, HEATING A PORTION OF THE COKE PARTICLES FROM THE COKING ZONE IN A HEATING ZONE TO INCREASE THE TEMPERATURE OF SAID FLUIDIZED PARTICLES, RETURNING A PORTION OF THE HEATED COKE PARTICLES FROM THE HEATING ZONE TO THE COKING ZONE AND WIRHDRDAWING COKE PRODUCT PARTICLES, WHICH COMPRISES THE STEPS OF CALCINING THE FLUID COKE PARTICLES TO A REAL DENSITY OF 1.87-1.92; GRINDING THE CALCINED FLUID COKE PARTICLES SO THAT 20-50 WEIGHT PERCENT OF THE FLUID COKE CHARGE HAS A DIAMETER OF LESS THAN 75 MICRONS; ADMIXING THE GROUND FLUID COKE WITH A CARBONACEOUS BINDER IN THE RATIO OF APPROXIMATELY 18-45 PARTS BY WT. PER 100 PARTS OF FLUID COKE; AND BAKUNG THE MUXTURE AT A TEMPERATURE IN THE RANGE OF 1700-2400*F. 